1. Field of the Invention
The present invention relates to a light emitting element using electroluminescence and a light emitting device provided with the light emitting element. The invention further relates to a method of manufacturing a light emitting element.
2. Description of the Related Art
In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed. In a basic structure of this light emitting element, a layer containing a light emitting organic compound is sandwiched between a pair of electrodes. By applying voltage to this element, electrons and holes are separately injected from the pair of electrodes into the layer containing a light emitting organic compound, and current flows. Then, recombination of these carriers (the electrons and holes) causes the light emitting organic compound to form an excited state and to emit light when the excited state returns to a ground state. With such a mechanism, such a light emitting element is referred to as a current-excitation light emitting element.
Note that an excited state of an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
Since the light emitting element is formed of an organic thin film, for example, with a thickness of approximately 0.1 μm, it is a great advantage of such a light emitting element that the light emitting element can be manufactured to be thin and lightweight. In addition, since time between carrier injection and light emission is approximately 1 μsec or less, the extremely high response speed is another advantage. These characteristics are considered suitable for a flat panel display element.
Such a light emitting element is formed in a film shape. Thus, surface emission can be easily achieved by forming a large-area element. This characteristic is hardly obtained in the case of a point light source typified by an incandescent lamp or an LED or a linear light source typified by a fluorescent lamp. Therefore, the above described light emitting element also has a high utility value as a surface light source which is applicable to lighting or the like.
As described above, a current-excitation light emitting element using the light emitting organic compound is expected to be applied to a light emitting device, lighting, and the like. However, there are still many issues. One of the issues is a reduction in power consumption. It is important to reduce drive voltage of the light emitting element in order to reduce power consumption. Since emission intensity of the current-excitation light emitting element depends on the amount of current flowing therethrough, it is necessary to make a large amount of current to flow at low voltage in order to reduce the drive voltage.
Meanwhile, as a structure of a light emitting element, a multiple quantum well structure is reported (Reference 1: Jingsong H, Kaixia Y, Shiyong L, and Hongjin J, Applied Physics Letters, Vol. 77, No. 12, 1750-1752, 2000). According to the Reference 1, it is considered that when a light emitting layer has a multiple quantum well structure, carriers are efficiently recombined and the luminous efficiency is increased.
Normally, a light emitting element is formed over a substrate provided with a thin film transistor (hereinafter referred to as a TFT) or the like. However, the process flow for forming a TFT is very complicated, and minute foreign bodies are easily generated; thus, it is difficult to remove the generated foreign bodies. In the case where the minute foreign bodies remain on the electrode, the film thickness of the element becomes nonuniform and a good light-emission cannot be obtained.
Further, as described in the Reference 1, the thickness of a layer of the multiple quantum well structure is as thin as approximately 3 nm. Accordingly, in the case where a layer of a multiple quantum well structure has minute foreign bodies, there may be a defect in the multiple quantum well structure. In other words, the multiple quantum well structure itself becomes difficult to be formed, and the effect of the multiple quantum well structure may not be obtained.
Further, in the case of forming the multiple quantum well structure, mainly vapor deposition is used. By controlling open/close of a shutter provided between an evaporation source and the substrate, the multiple quantum well structure may be formed. However, when the multiple quantum well structure is formed by controlling open/close of the shutter, the film formation takes more time and the throughput may be decreased.